Communications systems are frequently plagued with multipath transmission problems; that is, one signal portion is propagated directly from transmitter to receiver in a straight line/shortest-distance and time manner while other signal portions are propogated via reflecting objects resulting in varying delays in arrival times at the receiver.
Prior art systems have attacked the problem in various ways; some examples are described by Laksatos in U.S. Pat. No. 3,537,008. The solution of the Laksatos patent utilizes multiple phase synchronized carrier frequencies carrying a summed continuous pilot signal, a summed pulsed pilot signal and a summed message signal. A receiver is provided to separate the plurality of carrier signals, one from the other, and reconstruct the two pilot signals and the message signal by means of at least a series of filtering, delay measuring, and signal simulating circuits. No provision is made for recovery of the energy in the multipath transmission other than that energy traveling the direct (shortest) path from transmitter to receiver.
Most techniques for handling the multipath communications problem may be classified as diversity schemes. Receivers are used which are spaced geographically, in frequency, or in antenna polarization. The signals from the diverse receivers are then combined at a given point in the circuit to provide optimum signal to noise ratio or to maximize the sensitivity of the receiver complex.